The field of the invention is high precision probes that measure the distance to the surface of a test object. More particularly, the invention relates to high precision laser based measuring systems and associated optics and electronics to perform such distance determinations relating to the surface of a test object.
A preferred embodiment of the device is designed to be used with currently manufactured Coordinate Measuring Machines (CMM's). Coordinate Measuring Machines are used by many manufacturers, worldwide, to precisely determine if manufactured parts are in tolerance. The location of faces on parts, holes, screw threads, etc. can be precisely determined via the use of Coordinate Measuring Machines. Such machines usually have probes which must contact the surface of a part to be tested. A leading manufacturer of ruby tipped mechanical probes which physically contact test surfaces is Renishaw Electrical, Ltd. an English company.
In operation, the tip of the sensor is moved around in space by attaching it to the arms of a Coordinate Measuring Machine. The arms allow free travel of the sensor in all three dimensions and contain encoders which allow one to precisely determine the exact position of the probe tip in space. Typical resolution for a Coordinate Measuring Machine is 0.00001" or alternately 0.1 mil (2.5 micrometers).
Coordinate measurement is accomplished as follows. The three-dimensional part is mounted onto the measurement table. The arm of the Coordinate Measuring Machine is moved so that the tip of the contact probe comes down and touches the surface of the part. When the part is touched, the tip of the probe is deflected slightly and opens an electrical contact. When the contact is broken, encoders in the Coordinate Measuring Machine arms are electronically locked so that they maintain their readings even if the arm overshoots. A computer then reads the three axis coordinates where the part surface was encountered. By this means, the exact location of one point on an object, the X, Y, Z coordinate of the point in space, can be determined. Subsequent points on the part are measured in a similar manner until a sufficient number of points are measured, and the critical parameters of the part have been determined, to see if it is in tolerance.
A disadvantage of the contacting probe tip is that it does not work for flexible parts such as thin metal pieces, plastics, any kind of liquid, or other soft, deformable materials such as a foam product or clay, which could be deformed via the measurement process.
An additional disadvantage of the contacting method is that the probe tips typically have a small ruby sphere attached to the end. The diameter of the sphere is well known, so that its radius can be compensated for in the measurement process but, nonetheless, the radius of this sphere is inherently large (with 0.050" being an example). This means that an object with very fine detail cannot be measured by standard probe tips, because the probe tip is too large to reach into small features of complex objects. If smaller probe tips are made in order to extend into small crevices and similar areas, then the pressure from a sharpened tip that is exerted onto a part will be such as to severely dent or distort the part at the measurement point, giving false readings.
Contacting probes have another disadvantage. Because they mechanically contact the part, they have to be retracted away from the part before they can be moved laterally so that the probe tip is not dragged along the part surface.
All these disadvantages are overcome by the use of the disclosed laser-based, non-contact sensor system.